1. Field of the Invention
The present invention relates generally to mechanical inertial igniters, and more particularly to compact, low-volume, reliable and easy to manufacture mechanical inertial igniters and ignition systems for thermal batteries and the like used in munitions with relatively short duration firing setback acceleration (shock).
2. Prior Art
Thermal batteries represent a class of reserve batteries that operate at high temperature. Unlike liquid reserve batteries, in thermal batteries the electrolyte is already in the cells and therefore does not require a distribution mechanism such as spinning. The electrolyte is dry, solid and non-conductive, thereby leaving the battery in a non-operational and inert condition. These batteries incorporate pyrotechnic heat sources to melt the electrolyte just prior to use in order to make them electrically conductive and thereby making the battery active. The most common internal pyrotechnic is a blend of Fe and KClO4. Thermal batteries utilize a molten salt to serve as the electrolyte upon activation. The electrolytes are usually mixtures of alkali-halide salts and are used with the Li(Si)/FeS2 or Li(Si)/CoS2 couples. Some batteries also employ anodes of Li(Al) in place of the Li(Si) anodes. Insulation and internal heat sinks are used to maintain the electrolyte in its molten and conductive condition during the time of use. Reserve batteries are inactive and inert when manufactured and become active and begin to produce power only when they are activated.
Thermal batteries have long been used in munitions and other similar applications to provide a relatively large amount of power during a relatively short period of time, mainly during the munitions flight. Thermal batteries have high power density and can provide a large amount of power as long as the electrolyte of the thermal battery stays liquid, thereby conductive. The process of manufacturing thermal batteries is highly labor intensive and requires relatively expensive facilities. Fabrication usually involves costly batch processes, including pressing electrodes and electrolytes into rigid wafers, and assembling batteries by hand or semi-automatically. Other manufacturing processes have also been recently developed that are more amenable to automation. The batteries are encased in a hermetically-sealed metal container that is usually cylindrical in shape. Thermal batteries, however, have the advantage of very long shelf life of up to 20 years that is required for munitions applications.
Thermal batteries generally use some type of igniter to provide a controlled pyrotechnic reaction to produce output gas, flame or hot particles to ignite the heating elements of the thermal battery. There are currently two distinct classes of igniters that are available for use in thermal batteries. The first class of igniter operates based on electrical energy. Such electrical igniters, however, require electrical energy, thereby requiring an onboard battery or other power sources with related shelf life and/or complexity and volume requirements to operate and initiate the thermal battery. The second class of igniters, commonly called “inertial igniters”, operates based on the firing acceleration. The inertial igniters do not require onboard batteries for their operation and are thereby often used in high-G munitions applications such as in gun-fired munitions and mortars.
In general, the inertial igniters, particularly those that are designed to operate at relatively low firing setback or the like acceleration (shock) levels, have to be provided with the means for distinguishing events such as accidental drops or explosions in their vicinity from the firing acceleration levels above which they are designed to be activated. This means that safety in terms of prevention of accidental ignition is one of the main concerns in inertial igniters.
The need to differentiate accidental and other so-called no-fire events from the so-called all-fire event, i.e., the firing setback acceleration (shock) event necessitates the employment of a safety system which is capable of allowing initiation of the inertial igniter only when the inertial igniter is subjected to the impulse level threshold corresponding to or above the minimum all-fire impulse levels. The safety mechanism can be thought of as a mechanical delay mechanism, after which a separate initiation system is actuated or released to provide ignition of the inertial igniter pyrotechnics. An inertial igniter that combines such a safety system with an impact based initiation system and its alternative embodiments are described herein.
Inertia-based igniters must therefore comprise two components so that together they provide the aforementioned mechanical safety (delay mechanism) and to provide the required striking action to achieve ignition of the pyrotechnic element(s) of the inertial igniter. The function of the safety system (mechanism) is to hold the striker element fixed to the igniter structure until the inertial igniter is subjected to a high enough acceleration level with long enough duration, i.e., to a prescribed impulse level threshold, corresponding to the firing setback acceleration event. The prescribed impulse level threshold requirement is generally accompanied also with a minimum acceleration level requirement to ensure that the inertial igniter is safe, i.e., the striker element stays fixed to the inertial igniter structure, when subjected to relatively low acceleration levels for relatively long duration. Once the all-fire event, i.e., the said minimum acceleration level and the prescribed impulse level threshold has been reached, the said safety system (mechanism) releases the striker element, allowing it to accelerate toward its target. The ignition itself may take place as a result of striker impact, or simply contact or proximity. For example, the striker may be akin to a firing pin and the target akin to a standard percussion cap primer. Alternately, the striker-target pair may bring together one or more chemical compounds whose combination with or without impact will set off a reaction resulting in the desired ignition.
A schematic of a cross-section of a conventional thermal battery and inertial igniter assembly is shown in FIG. 1. In thermal battery applications, the inertial igniter 10 (as assembled in a housing) is generally positioned above the thermal battery housing 11 as shown in FIG. 1. Upon ignition, the igniter initiates the thermal battery pyrotechnics positioned inside the thermal battery through a provided access 12. The total volume that the thermal battery assembly 16 occupies within munitions is determined by the diameter 17 of the thermal battery housing 11 (assuming it is cylindrical) and the total height 15 of the thermal battery assembly 16. The height 14 of the thermal battery for a given battery diameter 17 is generally determined by the amount of energy that it has to produce over the required period of time. For a given thermal battery height 14, the height 13 of the inertial igniter 10 would therefore determine the total height 15 of the thermal battery assembly 16. To reduce the total volume that the thermal battery assembly 16 occupies within a munitions housing, it is therefore important to reduce the height of the inertial igniter 10. This is particularly important for small thermal batteries since in such cases the inertial igniter height with currently available inertial igniters can be almost the same order of magnitude as the thermal battery height.
The isometric cross-sectional view of a currently available inertia igniter is shown in FIG. 2, referred to generally with reference numeral 200. The full isometric view of the inertial igniter 200 is shown in FIG. 3. The inertial igniter 200 is constructed with igniter body 201, consisting of a base 202 and at least three posts 203. The base 202 and the at least three posts 203, can be integrally formed as a single piece but may also be constructed as separate pieces and joined together, for example by welding or press fitting or other methods commonly used in the art. The base 202 of the housing can also be provided with at least one opening 204 (with a corresponding opening(s) in the thermal battery—not shown) to allow ignited sparks and fire to exit the inertial igniter and enter into the thermal battery positioned under the inertial igniter 200 upon initiation of the inertial igniter pyrotechnics 215, or percussion cap primer when used in place of the pyrotechnics 215 (not shown). Although illustrated with the opening 204 in the base, the opening (or openings) can alternatively be formed in a side wall or in the striker mass as described in U.S. Patent Application Publication No. 2011/0171511 filed on Jul. 13, 2010, the entire contents thereof is incorporated herein by reference.
A striker mass 205 is shown in its locked position in FIG. 2. The striker mass 205 is provided with guides for the posts 203, such as vertical surfaces 206, that are used to engage the corresponding (inner) surfaces of the posts 203 and serve as guides to allow the striker mass 205 to ride down along the length of the posts 203 without rotation with an essentially pure up and down translational motion.
In its illustrated position in FIGS. 2 and 3, the striker mass 205 is locked in its axial position to the posts 203 by at least one setback locking ball 207. The setback locking ball 207 locks the striker mass 205 to the posts 203 of the inertial igniter body 201 through the holes 208 provided in the posts 203 and a concave portion such as a dimple (or groove) 209 on the striker mass 205 as shown in FIG. 2. A setback spring 210, which is preferably in compression, is also provided around but close to the posts 203 as shown in FIGS. 2 and 3. In the configuration shown in FIG. 2, the locking balls 207 are prevented from moving away from their aforementioned locking position by the collar 211. The setback spring 210 is preferably a wave spring with rectangular cross-section. The collar 211 is usually provided with partial guide 212 (“pocket”), which are open on the top as indicated by the numeral 213. The guide 212 may be provided only at the location of the locking balls 207 as shown in FIGS. 2 and 3, or may be provided as an internal surface over the entire inner surface of the collar 211 (not shown).
The collar 211 rides up and down on the posts 203 as can be seen in FIGS. 2 and 3, but is biased to stay in its upper most position as shown in FIGS. 2 and 3 by the setback spring 210. The guides 212 are provided with bottom ends 214, so that when the inertial igniter is assembled as shown in FIGS. 2 and 3, the setback spring 210 which is biased (preloaded) to push the collar 211 upward away from the igniter base 201, would “lock” the collar 211 in its uppermost position against the locking balls 207. As a result, the assembled inertial igniter 200 stays in its assembled state and would not require a top cap to prevent the collar 211 from being pushed up and allowing the locking balls 207 from moving out and releasing the striker mass 205.
In the inertial igniters of the type shown in FIGS. 2 and 3, a one part pyrotechnics compound 215 (such as lead styphnate or other similar compound) is used as shown in FIG. 2. The striker mass 205 is usually provided with a relatively sharp tip 216 and the igniter base surface 202 is provided with a protruding tip 217 which is covered with the pyrotechnics compound 215, such that as the striker mass 205 is released during an all-fire event and is accelerated down (opposite to the arrow 218 illustrated in FIG. 2), impact occurs mostly between the surfaces of the tips 216 and 217, thereby pinching the pyrotechnics compound 215, thereby providing the means to obtain a reliable initiation of the pyrotechnics compound 215. Alternatively, a two-part pyrotechnics compound consisting, for example, one being based on potassium chlorate used in place of the pyrotechnics 215 and the other based on red phosphorous which is positions over a (generally larger) tip 216 of the striker mass 206, may be used. In another alternative design, instead of using the pyrotechnics compound 215, FIG. 2, a percussion cap primer or the like (not shown) is used. In such inertial igniters, the tip 216 of the striker mass 205 is appropriately sized for initiating the percussion cap primer being used.
The basic operation of the inertial igniter 200 shown in FIG. 2 and is as follows. Any non-trivial acceleration in the axial direction 218 which can cause the collar 211 to overcome the resisting force of the setback spring 210 will initiate and sustain some downward motion of the collar 211. The force due to the acceleration on the striker mass 205 is supported at the dimples 209 by the locking balls 207 which are constrained inside the holes 208 in the posts 203. If an acceleration time in the axial direction 218 imparts a sufficient impulse to the collar 211 (i.e., if an acceleration time profile is greater than a predetermined threshold), it will translate down along the axis of the assembly until the setback locking balls 205 are no longer constrained to engage the striker mass 205 to the posts 203. If the acceleration event is not sufficient to provide this motion (i.e., the acceleration time profile provides less impulse than the predetermined threshold), the collar 211 will return to its start (top) position under the force of the setback spring 210.
Assuming that the acceleration time profile was at or above the specified “all-fire” profile, the collar 211 will have translated down past the locking balls 207, allowing the striker mass 205 to accelerate down towards the base 202. In such a situation, since the locking balls 207 are no longer constrained by the collar 211, the downward force that the striker mass 205 has been exerting on the locking balls 207 will force the locking balls 207 to move outward in the radial direction. Once the locking balls 207 are out of the way of the dimples 209, the downward motion of the striker mass 205 is no longer impeded. As a result, the striker mass 205 moves downward, causing the tip 216 of the striker mass 205 to strike the pyrotechnic compound 215 on the surface of the protrusion 217 with the requisite energy to initiate ignition.
In the inertial igniter 200 of FIGS. 2 and 3, following ignition of the pyrotechnics compound 215, the generated flames and sparks are designed to exit downward through the opening 204 to initiate the thermal battery below. Alternatively, if the thermal battery is positioned above the inertial igniter 200, the opening 204 can be eliminated and the striker mass could be provided with at least one (not shown) to guide the ignition flame and sparks up through the striker mass 205 to allow the pyrotechnic materials (or the like) of a thermal battery (or the like) positioned above the inertial igniter 200 to be initiated.
In the inertial igniter 200 of FIGS. 2 and 3, by varying the mass of the striker 205, the mass of the collar 211, the spring rate of the setback spring 210, the distance that the collar 211 has to travel downward to release the locking balls 207 and thereby release the striker mass 205, and the distance between the tip 216 of the striker mass 205 and the pyrotechnic compound 215 (and the tip of the protrusion 217), the designer of the disclosed inertial igniter 200 can match the all-fire and no-fire impulse level requirements for various applications as well as the safety (delay or dwell action) protection against accidental dropping of the inertial igniter and/or the munitions or the like within which it is assembled.
Briefly, the safety system parameters, i.e., the mass of the collar 211, the spring rate of the setback spring 210 and the dwell stroke (the distance that the collar 210 has to travel downward to release the locking balls 207 and thereby release the striker mass 205) must be tuned to provide the required actuation performance characteristics. Similarly, to provide the requisite impact energy, the mass of the striker 205 and the aforementioned separation distance between the tip 216 of the striker mass and the pyrotechnic compound 215 (and the tip of the protrusion 217) must work together to provide the specified impact energy to initiate the pyrotechnic compound when subjected to the remaining portion of the prescribed initiation acceleration profile after the safety system has been actuated.
In general, the required acceleration time profile threshold for inertial igniter initiation, i.e., the so-called all-fire condition, is described in terms of an acceleration pulse of certain amplitude and duration. For example, the all-fire acceleration pulse may be given as being 1000 G for 15 milliseconds. The no-fire (no-initiation) condition may be indicated similarly with certain acceleration pulse (or half-sine) amplitude and duration. For example, the no-fire condition may be indicated as being an acceleration pulse of 2000 G for 0.5 milliseconds. Other no-fire conditions may include transportation induced vibration, usually around 10 G with a range of frequencies.
It is appreciated by those skilled in the art that when the inertial igniter 200 of FIGS. 2 and 3 is subjected to the aforementioned all-fire acceleration profile threshold, the collar 211 is first caused to be displaced downward under the force caused by the acceleration in the direction of the arrow 218 acting on the inertia (mass) of the collar 211, until the striker mass 205 is released as was described above and accelerated downward to towards the base 202 of the inertial igniter until the tip 216 of the striker mass 205 strikes the pyrotechnic material 215 over the protruding tip 217 and causing it to ignite. It is also appreciated by those skilled in the art that the process of downward travel of the collar 211 takes a certain amount of time, hereinafter indicated as Δt1, the amount of which is dependent on the mass of the collar 211 and the aforementioned preloading level of the compressive spring 210 and the distance that it has to travel downward before the balls 207 and thereby the striker mass 205 is released. Similarly, once the striker mass 205 is released, the process of downward travel of the striker mass 205 until its tip 216 strikes the pyrotechnic material 215 over the protruding tip 217 takes a certain amount of time for, hereinafter indicates as Δt2, the amount of which is dependent on the level of acceleration in the direction of the arrow 218.
In addition, in recent years new improved chemistries and manufacturing processes have been developed that promise the development of lower cost and higher performance thermal batteries that could be produced in various shapes and sizes, including their small and miniaturized versions. However, inertial igniters are relatively large and not suitable for small and low power thermal batteries, particularly those that are being developed for use in miniaturized fuzing, future smart munitions, and other similar applications. This is general the case for munitions with relatively low firing setback acceleration, particularly those in which the firing setback acceleration pulse (shock) has relatively short duration.
It is therefore appreciated by those skilled in the art that the duration of the all fire acceleration must at least be the sum of the above two time periods Δt1 and Δt2, hereinafter indicated as Δt=Δt1+Δt2. For example, for the aforementioned case of all-fire (setback) acceleration being 1000 G for 15 milliseconds, the total time Δt must be less than the indicated acceleration duration of 15 milliseconds.
In certain applications, the aforementioned total time Δt is small enough that even by optimizing the parameters design of the inertial igniter of the type shown in FIGS. 2 and 3 to minimize the required aforementioned time periods Δt1 and Δt2, the required total time Δt cannot be reduced to below the all-fire acceleration period.
In certain other case, due to the small size or geometry of the thermal battery or the like, the height of the inertial igniter that can be used is so small that the striker mass 205 upon its release does not have enough distance to travel downward to gain enough velocity (i.e., enough kinetic energy) before its tip 216 strikes the pyrotechnic material 215 over the protruding tip 217 in order to be able to cause the pyrotechnic material 215 to be reliably ignited.